1. Field of the Invention
The present invention relates to a current driver and particularly to a technology of current drivers suitable as a display driver for a display device, such as an organic EL (electroluminescent) panel, and the like.
2. Description of the Prior Art
In recent years, in the fields of flat panel displays, such as organic EL panels, and the like, the screen size and definition have been increasing while the thickness, weight and production cost have been decreasing. In general, the active matrix method has been favorably employed as a method for driving a large, high-definition display panel. Hereinafter, a display driver for a conventional active matrix display panel is described.
FIG. 14 is a circuit diagram showing the structure of a display panel and a conventional current driver connected to the display panel. In the example of FIG. 14, the current driver is a display driver. The display panel is an organic EL panel.
Referring to FIG. 14, the conventional current driver includes current supply sections 1001a1, 1001a2, . . . and 1001an (hereinafter, referred to as “current supply section(s) 1001a” when generically mentioned) for supplying driving currents respectively to a plurality of pixel circuits 1005a1, 1005a2, . . . and 1005am (hereinafter, referred to as “pixel circuit(s) 1005a”, when generically mentioned) which are arranged in a matrix over the display panel, and a reference current supply section (bias circuit) 1101 for supplying the reference current to the current supply sections 1001a. In the present specification, the “reference current” means an electric current having a predetermined value, which is supplied from a reference current source. The “reference current” also means an electric current derived from the reference current source and transmitted by a current mirror circuit.
In the case of a device having a large size display panel, such as a television display device, a plurality of semiconductor chips (driver LSI chips) 1105 in which current supply sections 1001a having m output terminals are integrated are used for driving the display panel. In many cases, these semiconductor chips 1105 are aligned in a line at a peripheral portion of the display panel.
Each of the pixel circuits 1005a1, 1005a2, . . . and 1005am includes a first TFT (Thin Film Transistor) 1104 of p-channel type, which is connected to the current supply section 1001a through a signal line, a second TFT 1102, and an organic EL element 1103 which emits light according to an electric current supplied from the second TFT 1102. The first TFT 1104 and second TFT 1102 constitute a current mirror circuit.
The reference current supply section 1101 includes: a first MISFET 1108 of p-channel type, one end of which being supplied with a supply voltage; a resistor 1107 for generating a reference current, which is connected to the first MISFET 1108; a second MISFET 1109 of p-channel type; and a current input MISFET 1110 of n-channel type for transmitting the reference current to the current supply sections 1001a, which is connected to the second MISFET 1109. The first MISFET 1108 and second MISFET 1109 constitute a current mirror circuit. In the example of FIG. 14, the reference current supply section 1101 is provided outside the semiconductor chips 1105. However, the reference current supply section 1101 may be provided on the semiconductor chip 1105. In this specification, in an example where a plurality of semiconductor chips 1105 are provided in a display device, a semiconductor chip for supplying the reference current to the other semiconductor chips is referred to as “master chip” while the semiconductor chips which receive the reference current from the “master chip” are referred to as “slave chips”.
In a system where n-bit scale is controlled, each of the current supply sections 1001a includes current sources 1112-1, 1112-2, . . . and 1112-n (n is a positive integer) arranged in parallel to each other with respect to an output section that is connected to the pixel circuit 1005a, and switches 1115-1, 1115-2, . . . and 1115-n for controlling the on/off states of the electric current flowing through the current sources 1112-1, 1112-2, . . . and 1112-n. Herein, each of the current sources 1112-1, 1112-2, . . . and 1112-n is formed by an n-channel type MISFET. This n-channel type MISFET and the current input MISFET 1110 constitute a current mirror circuit. Each of the switches 1115-1, 1115-2, . . . and 1115-n independently carries out the switching operation according to display data.
With the above-described structure, the operation of a display device driven by an electric current is controlled.